Known examples of conventional systems that detect the air pressure of for example a tire of an automobile include a tire air pressure monitoring system (Tire Pressure Monitoring System: TPMS). In general, a TPMS is attached to the inside of a tire and is equipped with a sensor, which detects the air pressure and temperature of the tire, and a transmitter, which wirelessly transmits data to a transceiver located in the vehicle. Accordingly, in a TPMS, it is necessary that there be a power source for supplying electric power to the sensor and the transmitter for example.
A variety of power generators have been proposed to date as power sources that can be used in TPMS's and the like. Among these power generators, in particular, piezoelectric power generators that utilize the piezoelectric effect have been the focus of much attention since this kind of power generator can be easily reduced in size.
For example, the piezoelectric power generator illustrated in FIG. 24 is described in below-cited Patent Literature 1. As illustrated in FIG. 24, a piezoelectric power generator 100 includes an elastic body 102 that has an end portion on one side thereof fixed in place and has a weight 101 attached to an end portion on the other side thereof. Contacts 103 and 104 are provided on main surfaces 102a and 102b of the elastic body 102. In addition, piezoelectric elements 105 and 106 are fixed onto the main surfaces 102a and 102b. 
In the piezoelectric power generator 100, when the elastic body 102 is displaced in the upward direction due to a stress being applied to the weight 101, the contact 103 applies a pressure to the piezoelectric element 105. As a result, the piezoelectric element 105 is deformed and power is generated in the piezoelectric element 105. Furthermore, when the elastic body 102 is displaced in the downward direction due to a stress being applied to the weight 101, the contact 104 applies a pressure to the piezoelectric element 106. As a result, the piezoelectric element 106 is deformed and power is generated in the piezoelectric element 106.
Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2005-312269